This invention relates generally to current source circuits, and more particularly the invention relates to a current source circuit which can be operated from a low-voltage variable power supply and is independent of power supply voltage and start-up currents.
A current source circuit typically has a high output impedance but produces current which is not necessarily independent of the power supply voltage. Dobkin U.S. Pat. No. 3,930,172 discloses a circuit which is independent of power supply. As described in the patent, a first pair of transistors is connected in series with one another between the supply and a second pair of transistors is connected in series with one another between the supply. The base-emitter junctions of the transistors are connected in a series loop, such that a voltage is developed between the base-emitter junctions of two adjacent transistors which is equal to the base-emitter voltage summation. The base-emitter voltages of any series-connected transistors oppose one another in the series loop. Since the collector currents of any series-connected transistors are equal to one another and their base-emitter voltages oppose one another in the series loop, the base-emitter voltage summation is independent of collector currents and, therefore, independent of the input supply. However, the Dobkin circuit requires voltage equal to two base-emitter voltage drops (V.sub.BE) for minimum operation. The output resistance is actually negative.
Taylor U.S. Pat. No. 4,574,233 discloses a circuit including a first transistor having an output current which is sensed across a resistance connected between the emitter of the first transistor and the negative side of a supply voltage. A series-negative feedback loop comprising two transistors is connected between the emitter of the first transistor and the base of the first transistor. The three transistors and the other circuit components are selected to result in an incremental output resistance approaching that of a cascode current source, while having a voltage drop across the circuit of substantially less than one volt. However, the output current through the first transistor is dependent on input currents through the pair of transistors which are in turn dependent on the power supply.